New York: It was an idea born of noble intentions: to help more centrist politicians get elected and reduce, in the words of one Newsweek columnist, the number of “jackasses” in office.

But an electoral experiment in America’s most populous and progressive state is now threatening to lock Democrats out of several potentially winnable congressional races and, in a nightmare scenario for the party, deny them a majority in the US House of Representatives in November.

California — the home state of Richard Nixon and Ronald Reagan — was once a reliably Republican state, but is now a Democratic stronghold, thanks largely to the growing number of immigrants, especially Hispanics. Donald Trump received just 32 per cent of the vote in California in 2016, the lowest of any major party candidate since 1924.

The loathing most Californians feel towards their president has fuelled Democratic hopes they could flip up to seven seats in California in November, more than in any other state.

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That enthusiasm, ironically, could lead to disaster for the party in Tuesday’s primaries. A glut of left-leaning candidates is running for office and Democrats fear they could cancel each other out, leaving only Republicans on the ballot in November in several crucial swing seats.

The situation is particularly fraught because the House contest has tightened dramatically in recent months as Republicans rally around President Trump, transforming a likely Democratic landslide into a toss-up.

Anti-Trump sentiment has left Democrats tripping over themselves to line-up against his policies. Credit:AP

A May CNN poll found the Democrats' advantage in the "generic ballot" - which measures support among the two parties - had plummeted from 16 points in February to just three points now.

The potential for calamity in California is possible because of the state's quirky way of determining who gets on the ballot: the so-called "top two" or "jungle" primary system.

In 2010 Californians approved a new system in which the two candidates with the most primary votes, regardless of party affiliation, advance to the general election. This replaced the system, still used in almost every other state, where Democrats and Republicans hold separate primaries, guaranteeing candidates from both parties a spot on the ballot.

Given the state's increasingly progressive nature, jungle primaries should work in the Democrats' favour and they sometimes have - like in 2016 when two California Democrats, and no Republicans, were on the Senate ballot.

But Democrats are petrified the opposite will happen this year if the party cannibalises its own vote.

Rob Pyers, research director at California Target Book, a subscription service targeted at California political insiders, told Fairfax Media there is "a very real threat" that Democrats could be locked out of winnable contests.

He said the Democrats' disorganisation was partly to blame for the mess.

"When the field was growing to upwards of 10 Democrats running against a single Republican, simple math should have pointed to the potential danger in the event additional Republicans jumped into the race," he said.

Mr Pyers said Democrats are also disadvantaged because turnout in non-presidential election years is usually higher among older, whiter and more conservative voters.

Democrats are especially worried about their prospects in three Republican-held seats classified as highly competitive by independent ratings agencies: California's 39th, 48th and 49th districts.

The current representatives of the 39th and 49th districts, Darrell Issa and Ed Royce, announced last year they would not recontest, prompting a flood of Democrats to put up their hand.

Polling conducted in March by Fight Back California, a pro-Democrat Political Action Committee, found the Democrat contenders in these districts were splitting the party vote, leading to the possibility two Republicans could sneak onto the ballot.

“The top two primary math is working against Democrats in these districts,” the PAC's chief strategist Katie Merrill said.

In the 48th district, in affluent Orange County, the stridently conservative and pro-Putin congressman Dana Rohrabacher was seen as highly vulnerable given his district voted for Hillary Clinton in 2016.

In that district, Harley Rouda, a real estate investor and former Republican, is competing against stem-cell researcher Hans Keirstead for the Democrat nomination.

Pro-Democrat PACs have spent millions of dollars on advertising in recent weeks in a bid to channel support to the leading Democratic candidates while suppressing the Republican vote. Democrats are also being implored not to waste their vote on "ghost" candidates who have withdrawn but whose names remain on the ballot.

Even if they secure a place on the ballot, the jungle primaries have exposed a deeper problem for California Democrats: a lack of experienced, high-profile candidates who can draw widespread support.

President Trump has tried to energise his supporters in recent weeks by warning about the dangers of losing control of the House. “We cannot get complacent," he told a National Rifle Association gathering last month. "We have to win the midterms.”

Matthew Knott is a Fairfax Media reporter based in the United States. He previously worked in the Canberra press gallery and recently finished a Masters of Journalism at Columbia University in New York.